The Universal Mobile Telecommunication System (UMTS) is one of the third generation mobile communication technologies designed to succeed GSM. 3GPP Long Term Evolution (LTE) is a project within the 3rd Generation Partnership Project (3GPP) to improve the UMTS standard to cope with future requirements in terms of improved services, such as higher data rates, improved efficiency, and lowered costs. The Universal Terrestrial Radio Access Network (UTRAN) is the radio access network of a UMTS and Evolved UTRAN (E-UTRAN) is the radio access network of an LTE system. In an E-UTRAN, a user equipment (UE) 150 is wirelessly connected to a radio base station (RBS) 110a, commonly referred to as an evolved NodeB (eNodeB), as illustrated in FIG. 1. Each eNodeB 110a-c serves one or more areas, referred to as cells 120a-c. In FIG. 1, a link between two nodes, such as the link between a positioning node here called Evolved Serving Mobile Location Center (E-SMLC) 100 and an eNodeB 110a,b,c, may be either a logical link, e.g. via higher-layer protocols and/or via other nodes, or a direct link. Hereinafter, a UE in a positioning architecture is a general term covering a positioning target which may, e.g., be a mobile device, a laptop, a small radio node or base station, a relay, or a sensor. A radio base station is a general term for a radio network node capable of transmitting radio signals. A radio base station may, e.g., be a macro base station, a micro base station, a home eNodeB, a beaconing device, or a relay.
UE positioning is a process of determining UE coordinates in space. Once the coordinates are available, they may be mapped to a certain place or location. The mapping function and delivery of the location information on request are parts of a location service which is required for basic emergency services. Services that further exploit a location knowledge or that are based on the location knowledge to offer customers some added value are referred to as location-aware and location-based services. The possibility of identifying a UE's geographical location has enabled a large variety of commercial and non-commercial services such as navigation assistance, social networking, location-aware advertising, and emergency calls, among others. Different services may have different positioning accuracy requirements imposed by an application. Furthermore, requirements on the positioning accuracy for basic emergency services defined by regulatory bodies exist in some countries. An example of such a regulatory body is the Federal Communications Commission (FCC) regulating the area of telecommunications in the United States.
There exist a variety of positioning techniques in wireless communications networks, differing in their accuracy, implementation cost, complexity, and applicability in different environments. Positioning methods may be broadly categorized into satellite based and terrestrial methods. Global Navigation Satellite System (GNSS) is a standard generic term for satellite navigation systems that enable UEs to locate their position and acquire other relevant navigational information. The Global Positioning System (GPS) and the European Galileo positioning system are well known examples of GNSS. In many environments, the position may be accurately estimated by using positioning methods based on GPS. Nowadays wireless networks also often have a possibility to assist UEs in order to improve an UE receiver sensitivity and a GPS start up performance, as for example in the Assisted-GPS (A-GPS) positioning method. However, GPS or A-GPS receivers are not necessarily available in all wireless UEs, and some wireless communications systems do not support A-GPS. Furthermore, GPS-based positioning may often have unsatisfactory performance in urban and/or indoor environments. There may therefore be a need for a complementary terrestrial positioning method.
There are a number of different terrestrial positioning methods. Some examples are:
Cell Identity (CID) based positioning, where the location is based on the identity of the current cell. Enhanced CID (E-CID) also takes e.g. Timing Advance (TA) into account to improve the positioning accuracy which may be important for positioning in large cells.
UE-based and UE-assisted Observed Time Difference Of Arrival (OTDOA), where the UE position is determined based on UE measurements of reference signals from three or more sites or locations.
Network based Uplink Time Difference Of Arrival (U-TDOA) positioning, where the UE position is determined based on several RBS measurements of a reference signal transmitted by the UE. Multi-lateration is then used to find a UE position as the intersection of hyperbolas when based on time difference measurements, or of circles when based on time of arrival measurements.
Fingerprinting or pattern matching positioning, where location fingerprints are collected in an off-line phase and are used for mapping measured signal strengths with a position. Location fingerprints are e.g. vectors of signal strength values of reference signals received from different RBSs in a position. Adaptive E-CID (AECID) is a fingerprinting positioning method that combines geographical cell descriptions corresponding to CIDs, received signal strengths and TA. AECID may also be extended to include Angle of Arrival (AoA) information. Whenever an A-GPS, A-GNSS or OTDOA high precision positioning is performed, the E-SMLC orders measurements of the radio properties which is a subset of geographical cell descriptions, TA, signal strengths and AoA. The radio property measurements are quantized and produce the fingerprint of the obtained high precision position.
Positioning methods based on time difference of arrival (TDOA) measurements have been widely used, for example in GSM, UMTS and cdma2000. For LTE networks, UE-assisted OTDOA positioning, which is based on downlink TDOA measurements, is currently being standardized. A corresponding UE-based mode is another possible candidate for later releases. The UE-assisted and UE-based modes differ in where the actual position calculation is carried out.
In the UE-assisted mode, the UE measures the TDOA of several cells and sends the measurement results to the network. A positioning node or a location server in the network carries out a position calculation based on the measurement results. In LTE, the positioning node in the control plane is referred to as an E-SMLC. The E-SMLC 100 is either a separate network node, as illustrated in FIG. 1, or a functionality integrated in some other network node. In the UE-based mode, the UE makes the measurements and also carries out the position calculation. The UE thus requires additional information for the position calculation, such as a position of the measured RBSs and a timing relation between the RBSs. In the user plane, the location or positioning node is referred to as Secure User Plane Location (SUPL) Location Platform (SLP).
The OTDOA positioning has won good acceptance among operators and vendors for LTE positioning. Some operators have already started to plan for OTDOA deployment in the LTE system. Moreover, the OTDOA related protocol in E-UTRAN has been adopted by the Open Mobile Alliance for user plane positioning. OTDOA is already standardized by 3GPP for GSM/EDGE RAN and UTRAN, but is not yet deployed in operational networks.
The OTDOA positioning is a multi-lateration technique measuring TDOA of reference signals received from three or more sites 210a-c (see FIG. 2a). To enable positioning, the UE should thus be able to detect positioning reference signals from at least three geographically dispersed RBS 210a-c with a suitable geometry, as the UE's position may be determined by the intersection 230 of at least two hyperbolas 240. This implies that the reference signals need to be strong enough or to have high enough signal-to-interference ratio in order for the UE to be able to detect them. With the OTDOA technique, the UE's position may be figured out based on the following measured parameters:
TDOA measurements of downlink reference signals;
Actual Relative Time Difference (RTD) between the RBS transmissions at the time when TDOA measurements are made;
Geographical position of the RBS whose reference signals are measured.
With more or longer TDOA measurements for each RBS, a better accuracy may be obtained. Measuring TDOA for signals from more than three RBSs typically also improves the positioning accuracy, although additional inaccurate measurements may also degrade the final accuracy. The accuracy of each of the measurements thus contributes to the overall accuracy of the position estimate.
There are several approaches to how to determine the RTD. One is to synchronize transmissions of the RBSs, as is generally done in a system using Time Division Duplex. In this case, RTD is a known constant value that may be entered in a database and used when calculating a position estimate. The synchronization must be done to a level of accuracy of the order of tens of nanoseconds in order to get an accurate position estimate. Ten nanoseconds uncertainty corresponds to three meters of error in the position estimate. Drift and jitter in the synchronization timing must also be well-controlled as they also contribute to the uncertainty in the position estimate. Synchronization to this level of accuracy is currently readily available through satellite based time-transfer techniques. Another alternative is to leave the RBSs to run freely without synchronization but within some constraint on the maximum frequency error. In this scenario, the RTD will change with time. The rate of change will depend on the frequency difference and jitter between RBSs.
LTE Positioning Protocol (LPP) and LTE Positioning Protocol annex (LPPa) are protocols used for carrying out OTDOA in a control plane solution in LTE. When receiving a positioning request for the OTDOA method, the E-SMLC requests OTDOA-related parameters from eNodeB via LPPa. The E-SMLC then assembles and sends assistance data and the request for the positioning to the UE via LPP. FIGS. 3a-d illustrate example architectures and protocol solutions of a positioning system in an LTE network. In the control plane solution, illustrated in FIG. 3a, the UE communicates with the E-SMLC transparently via the eNodeB and the Mobility Management Entity (MME) over LPP, and the eNodeB communicates with the E-SMLC transparently via the MME over LPPa. The user plane solution illustrated in FIG. 3b does not rely on the LPPa protocol, although 3GPP allows for the possibility of inter-working between the control and user plane positioning architectures. The SLP is the positioning node for user-plane positioning, similar to E-SMLC for control plane positioning, and there may or may not be an interface between the two positioning servers.
Since signals from multiple distinct sites need to be measured for OTDOA positioning, the UE receiver may have to deal with signals that are much weaker than those received from a serving cell. Furthermore, without an approximate knowledge of when the measured signals are expected to arrive in time and what is the exact pattern of a positioning reference signal, the UE would need to do signal search blindly within a large search window which would impact the accuracy of the measurements, the time it takes to perform the measurements, as well as the UE complexity. Therefore, to facilitate UE positioning measurements, the wireless network transmits assistance data to the UE. The assistance data and its quality are important for both the UE-based and the UE-assisted mode, although assistance data contents may differ for the two modes. The standardized assistance data includes among others a neighbor cell list with physical cell identities, a number of consecutive downlink sub frames used for the reference signals, an expected timing difference, and a search window. The expected timing difference and the search window, together referred to as search parameters, are crucial for an efficient correlation peak search.
According to the current 3GPP standard specifications, E-SMLC shall facilitate the expected OTDOA measurements at the UE side by providing the search window, allowing the UE to speed up the measurements and to keep a reasonable level of complexity. The quality or size of this search window impacts both the response time and the measurement accuracy and is therefore very important. With a narrower search window, the signal search performed by the UE is easier, although there is a higher risk of missing the correct signal peak if the search window has been derived with a lower confidence level.
The current 3GPP definition in LPP of the estimated timing difference, referred to as an expected Reference Signal Time Difference (RSTD), and of the search window, referred to as the expected RSTD Uncertainty, are:
Expected RSTD INTEGER (−8192 . . . 8191),
Expected RSTD-Uncertainty INTEGER (0 . . . 10234)
The resolution for both ExpectedRSTD and ExpectedRSTD-Uncertainty is 3×Ts, where Ts=1/(15000*2048) seconds according to the 3GPP specifications. For the reference cell and the measured neighbor cell operating on the same frequency, this corresponds to the search window [−ExpectedRSTD−Uncertainty×3×Ts, Expected RSTD-Uncertainty×3×Ts] centered at TREF+ExpectedRSTD×3×Ts, where TREF is the reception time of the beginning of the reference signal positioning occasion of the reference cell at the UE antenna connector.
The search window is defined as a symmetric range around the expected RSTD, and the expected RSTD uncertainty is the absolute value of the limits of the search window. If the search window is +/−30 μs, the corresponding RSTD uncertainty is 30 μs. A reduction of the uncertainty thus results in a smaller search window. The OTDOA measurement at the UE side is defined as a RSTD measurement in the 3GPP standard. RSTD is a relative timing difference between a neighbor cell and a reference cell. If T_SubframeRxNeighbor is a time when a target UE receives a start of a subframe from this neighbor cell, and T_SubframeRxRef is the time when the target UE receives the start of a subframe from the reference cell, the RSTD is equal to T_SubframeRxNeighbor−T_SubframeRxRef. In case of a cell geometry as the one illustrated in FIG. 2b, the time of the signal traveling from the radio node transmitter to the UE receiver in the reference cell 220, which is not necessarily a serving cell, gives a circle. For the sake of illustration, it may be assumed that the signal traveling time corresponds to the distance between the radio node, e.g. the eNodeB and the UE divided by the speed of light c. With such a cell geometry the maximum RSTD (RSTD_max) and minimum RSTD (RSTD_min) correspond to the rightmost and leftmost positions 250a, 250b, of the UE respectively. In this example, the search window may thus be deduced as follows:
                              RSTD_min          =                    ⁢                                                    T                                  bs                  ⁢                                                                          ⁢                  1                                            -                                                d                  ⁢                                                                          ⁢                  2                                c                            -                              (                                                      T                                          bs                      ⁢                                                                                          ⁢                      2                                                        -                                                            d                      ⁢                                                                                          ⁢                      1                                        c                                                  )                                      =                                                            (                                                            T                                              bs                        ⁢                                                                                                  ⁢                        1                                                              -                                          T                                              bs                        ⁢                                                                                                  ⁢                        2                                                                              )                                +                                                                            d                      ⁢                                                                                          ⁢                      1                                        -                                          d                      ⁢                                                                                          ⁢                      2                                                        c                                            =                                                Δ                  ⁢                                                                          ⁢                  T                                +                                                                            d                      ⁢                                                                                          ⁢                      1                                        -                                          d                      ⁢                                                                                          ⁢                      2                                                        c                                                                    ⁢                                  ⁢                  RSTD_max          =                    ⁢                                                    T                                  bs                  ⁢                                                                          ⁢                  1                                            -                                                d                  ⁢                                                                          ⁢                  4                                c                            -                              (                                                      T                                          bs                      ⁢                                                                                          ⁢                      2                                                        -                                                            d                      ⁢                                                                                          ⁢                      3                                        c                                                  )                                      =                                                            (                                                            T                                              bs                        ⁢                                                                                                  ⁢                        1                                                              -                                          T                                              bs                        ⁢                                                                                                  ⁢                        2                                                                              )                                +                                                                            d                      ⁢                                                                                          ⁢                      3                                        -                                          d                      ⁢                                                                                          ⁢                      4                                                        c                                            =                                                Δ                  ⁢                                                                          ⁢                  T                                +                                                                            d                      ⁢                                                                                          ⁢                      3                                        -                                          d                      ⁢                                                                                          ⁢                      4                                                        c                                                                                        [        1        ]            
where c is the speed of light, Tbs1 and Tbs2 are the System Frame Number (SFN) initialization time for RBS bs1, which is the reference RBS, and for RBS bs2, which is the neighbor RBS, respectively. Furthermore, d1 is the distance between the neighbor RBS bs2 and the UEs leftmost position 250b, d2 is the distance between the reference RBS bs1 and the UEs leftmost position 250b, d3 is the distance between the neighbor RBS bs2 and the UEs rightmost position 250a, and d4 is the distance between the reference RBS bs1 and the UEs rightmost position 250a, as illustrated in FIG. 2b. ΔT=Tbs1−Tbs2 is the SFN initialization time difference between the reference cell or RBS bs1 and the neighbor cell or RBS bs2.
In the equations [1], the distances needed for deducing the search window may be obtained from measurements. The distance between the UE and the reference cell may be estimated from a TA value. If this distance is denoted d (d2=d3=d), then, as follows from equations [1], the search window is the largest when d1=ISD−d and d3=ISD+d, where ISD is the absolute neighbor-to-reference eNodeB distance which is typically known to a positioning node. Thus, for a UE located at distance d from the reference cell, equations [1] gives
                              RSTD_min          =                                    Δ              ⁢                                                          ⁢              T                        +                                                            I                  ⁢                                                                          ⁢                  S                  ⁢                                                                          ⁢                  D                                -                                  2                  ⁢                  d                                            c                                      ⁢                                  ⁢                  RSTD_max          =                                    Δ              ⁢                                                          ⁢              T                        +                                          I                ⁢                                                                  ⁢                S                ⁢                                                                  ⁢                D                            c                                                          [        2        ]            
The UE may thus be informed that the estimated RSTD is equal to
            Δ      ⁢                          ⁢      T        +                            I          ⁢                                          ⁢          S          ⁢                                          ⁢          D                -        d            c        ,which corresponds to the center of the search window, and that the search window is given by [−d/c, d/c]. In this expression, d/c is the uncertainty signaled to the UE, which is the value that defines the search window or the range [−d/c,d/c]. The same applies for cells with different cell ranges since it is only the distance between the UE and the reference cell that matters.
In the current LTE standard, it has been specified that the maximum allowed expected RSTD uncertainty is five microseconds, which corresponds to a distance of 1.5 kilometers. The search window should thus not be larger than [−5 μs, 5 μs] or expressed in kilometers [−1.5 km, 1.5 km]. There is no clear description on how such a limited value of the expected RSTD uncertainty should be reached. It is well accepted that a cell ID based positioning may be used as a basis for deriving the uncertainty. This would provide an uncertainty which is proportional to the cell size, which may be enough in the case of a cell range smaller than 1.5 km.
However, cell coverage may be very big in a wireless network. Theoretically, the LTE standard supports a maximum cell range of 100 km. Providing a search window based on such a big uncertainty is not efficient enough for OTDOA measurements. Even though the cell ranges in operational networks are likely to be much smaller than the maximum cell range defined by the standard, they may not always be 1.5 km or smaller. Performing the measurements of reference signals with a too narrow search window on the other hand would result in that the searched reference signal peaks fall outside the search window with a high probability and either false peaks are detected or the reference signal detection fails.